As the important basic petrochemical raw materials, ethylene and propylene are of important symbolic significance of a country's economic development level. At present, the world's industrial production equipment of ethylene and propylene mainly adopts the steam pyrolysis of the hydrocarbons such as the naphtha and light diesel oil. In such way, the resulted pyrolysis gas is of the mixture which contains the hydrogen, methane, ethane, ethylene, propane, propylene, mixed C4 and C5, and pyrolysis gasoline, etc. and needs to be further separated and refined to produce the qualified chemicals such as the ethylene and propylene.
So far there has been no industrialized device running for preparing the polymer grade low-carbon olefin through the separation of methanol pyrolysis gas at home and abroad. The similar process is the deep cooling separation technology of the steam pyrolysis gas of the naphtha, etc. After multistage compression, in general a 5-stage compression, the pyrolysis gas is up to about 3.7 MPa and then enters the rectification separation system. According to the rectification separation sequence of the hydrocarbons, the rectification separation process is divided into such typical ethylene separation processes as sequential separation process (Lummus/Technip/KBR), front-end deethanizing process (Linde), and front-end depropanizing process (S&W). For the sequential separation process, the latest progress of the Lummus's ethylene separation technology includes the improvement of the compression refrigeration system (3-stage compression medium-pressure pyrolysis and binary/ternary refrigeration system), catalytic rectification hydrogenation (C3 selective hydrogenation), and olefins conversion in which the propylene is produced through the reaction of 2-butene with ethylene. The Technip sequential separation process adopts the 5-stage compression, dual demethanizing columns, and back-end C2 and C3 hydrogenation, and its latest gradual separation technology uses the pinch technology with the minimum energy consumption for the fuzzy separation, and the feedforward control system which is easy to be operated. The KBR separation process adopts the 5-stage compression, 4-stage outlet alkaline scrubbing, front-end high-pressure demethanizing column, back-end C2 and C3 hydrogenation. The front-end deethanizing process adopts the front-end C2 hydrogenation and is applicable for the gas containing a large number of C3+, represented by the Linde process; it is divided into the high-pressure process (3.3 MPa) and low-pressure process (1.18 MPa) depending on the operating pressure of the demethanizing column. For the front-end depropanizing process, after a 3-stage compression of the pyrolysis gas, the fractions lighter than C3 and heavier than C4 are separated, and the acetylene removal is available with the front-end hydrogenation, this process is applicable for the gas containing a large number of C4+. The S&W's latest technical advances include the viscosity control technology of the quenching oil and the HRS cold box patent technology.
The Chinese patent CN1157280A disclosed an energy-saving method for the separation of light hydrocarbon, improving the twin-column front-end deethanizing separation process. This patent provides energy saving through the improved feeding heat exchange mode in the demethanizing column.
The high price of the crude oil at the international market results in the shortage of the naphtha resource. The Methanol to Olefins (hereinafter referred to as MTO) process in which methanol is used as the raw material and then directly transformed into mixed low-carbon olefin through catalytic reaction has been in rapid development in recent years. The MTO's main products include the ethylene and the propylene at a ratio of 0.8 to 1.5 which increases with the increased reaction intensity. The composition of the MTO pyrolysis gas is significantly different from that of the pyrolysis gas of the naphtha etc., mainly embodied in the propylene and propane contained in the MTO pyrolysis gas significantly higher than those in the naphtha pyrolysis gas. If the conventional front-end deethanizing separation process is used for separating the MTO pyrolysis gas, high content of the propylene and propane in the MTO pyrolysis gas will result in the increased power consumption in the 5-stage compression. In addition, such conventional process provides high pressure after the 5-stage compression, and in order to avoid the autoclave in the deethanizing column from overhigh temperature which will lead to the diolefine polymerization, the high- and low-pressure dual columns are generally used for deethanizing. This process is long and complicated, resulting in increased investment. Furthermore, such conventional process, as disclosed in CN1157280A, in general adopts the low-pressure demethanizing technology requiring low temperature of the cold box at the location of the front-end dehydrogenation, which is applicable for the naphtha pyrolysis gas. In the MTO pyrolysis gas, however, the gas impurities contained such as the nitrogen oxides (NOX) and oxygen will lead to the accumulation of the dangerous explosives at the location of the cold box and thus increase insecurity factors of the system.